.Net: Align the behavior of the ChatMessageContent.Content property s…

…etter with streaming counterpart (#6753)

### Motivation and Context
This is a follow-up fix for the `ChatMessageContent.Content` property
setter, similar to the one that was done to the
`StreamingChatMessageContent.Content` property when addressing the PR
comment:
#6449 (comment).

### Description
This change updates the text of the first text item regardless of the
value being set. This helps avoid inconsistency that occurs when text
content already exists in the items collection, null is set via the
Content property, and the setter does nothing, while the getter returns
the text message content:
```csharp
var items = new ChatMessageContentItemCollection
{
    new TextContent("Hi AI."),
};

var chatMessage = new ChatMessageContent(AuthorRole.User, items: items);
chatMessage.Content = null; // The property setter does nothing if the value is null.

// Fails
Assert.Null(chatMessage.Content); // The property getter returns the "Hi AI." text.
```

dotnet/src/SemanticKernel.Abstractions/Contents/ChatMessageContent.cs

@@ -41,18 +41,12 @@ public class ChatMessageContent : KernelContent
         }
         set
         {
-            if (value is null)
-            {
-                return;
-            }
-
             var textContent = this.Items.OfType<TextContent>().FirstOrDefault();
             if (textContent is not null)
             {
                 textContent.Text = value;
-                textContent.Encoding = this.Encoding;
             }
-            else
+            else if (value is not null)
             {
                 this.Items.Add(new TextContent(
                     text: value,

dotnet/src/SemanticKernel.UnitTests/Contents/ChatMessageContentTests.cs

@@ -55,8 +55,10 @@ public void ContentPropertySetterShouldAddTextContentToItemsCollection()
         Assert.Contains(sut.Items, item => item is TextContent textContent && textContent.Text == "fake-content");
     }
 
-    [Fact]
-    public void ContentPropertySetterShouldUpdateContentOfFirstTextContentItem()
+    [Theory]
+    [InlineData(null)]
+    [InlineData("fake-content-1-update")]
+    public void ContentPropertySetterShouldUpdateContentOfFirstTextContentItem(string? content)
     {
         // Arrange
         var items = new ChatMessageContentItemCollection
@@ -68,10 +70,23 @@ public void ContentPropertySetterShouldUpdateContentOfFirstTextContentItem()
 
         var sut = new ChatMessageContent(AuthorRole.User, items: items)
         {
-            Content = "fake-content-1-update"
+            Content = content
+        };
+
+        Assert.Equal(content, ((TextContent)sut.Items[1]).Text);
+    }
+
+    [Fact]
+    public void ContentPropertySetterShouldNotAddTextContentToItemsCollection()
+    {
+        // Arrange
+        var sut = new ChatMessageContent(AuthorRole.User, content: null)
+        {
+            Content = null
         };
 
-        Assert.Equal("fake-content-1-update", ((TextContent)sut.Items[1]).Text);
+        // Assert
+        Assert.Empty(sut.Items);
     }
 
     [Fact]